UVI Spatial Analysis

Spatial patterns of UV-I observed by both data sources were also compared at a 4 x 4 km spatial resolution. Figures 10.4 -10.7 collectively delineate the comparison of seasonal UV-I spatial patterns between ground-based measurements and TOMS satellite data. Figure 10.4 first summarizes these spatial patterns of the ground surface UV-I data by season. The two data sources portray very similar spatial distributions of springtime UV-I (Fig. 10.4). The maximum of the UV-I is 9 for the USDA ground-based measurements during spring. The region between the south of New Mexico and Texas and the southern tip of Florida, where the UV-I values exceed 8, accounts for 4.5% of the total study area. However, using the TOMS satellite data, the maximum of the UV-I in spring is 10.3, and the region where the UV-I values exceed 8 accounts for 13.6% of the total study area. This region covers almost all the southern states in the Gulf of Mexico up to the southern Colorado plateau. The minimum UV-I in spring is 3.4. Using the USDA ground-based measurements, the region surrounding the northern states and the Great Lakes, where the UV-I values are less than 4, accounts for 5.4% of the total study area, whereas, using the TOMS data, it accounts for 9.8% of the total study area. Overall, the means of the USDA ground-based measurements and the TOMS data are 5.8 and 6.1, respectively. The lower ground-based measurements could be attributed to the fact that they are interpolated from the point data, and are thus constrained to range between the values observed at the stations. The standard deviations of the UV-I values based on these two data sources are 12.15 and 15.64, respectively. From the minimum and maximum values associated with both data sources, it can be concluded that the TOMS data may exhibit more versatile spatial patterns in response to the terrain complexity. Due to the restrictions of surface observations, the UV-I spatial distribution based on the USDA ground-based measurements, shows less sensitivity in response to topographic features and terrain complexity. This is especially true in the Colorado Plateau.

(a) USDA ground-based measurements

Figure 10.4 Map of the UV-I spatial distributions in spring based on (a) USDA ground-based measurements and (b) TOMS data

Figure 10.4 Map of the UV-I spatial distributions in spring based on (a) USDA ground-based measurements and (b) TOMS data

In contrast, the UV-I spatial distribution maps associated with the two data sources in summer (see Fig. 10.5) do not portray similar patterns across the continental U.S. In summary, the maximum summertime UV-I is 10.6 with the USDA ground-based measurements. The area where the UV-I values exceed 10 accounts for 3.4% of the total study area. It spreads from the south of New Mexico and Texas, but is absent from the southern tip of Florida. The maximum of the UV-I in summer is 12 with the TOMS satellite data. The area where the UV-I values exceed 10 accounts for 20.9% of the total study area. It spreads from the

(a) USDA groLind-hascd measurements

Figure 10.5 Map of the UV-I spatial distributions in summer based on (a) USDA ground-based measurements and (b) TOMS data

Figure 10.5 Map of the UV-I spatial distributions in summer based on (a) USDA ground-based measurements and (b) TOMS data

Colorado Plateau to southern Texas. The minimum from the USDA ground-based measurements data in summer is 5.4 and the area where the UV-I values are less than 6 accounts for 2.7% of the total study area. On the other hand, the minimum of TOMS satellite data in summer is 5.6 and the area where the UV-I values are less than 6, located mostly in the northern part of Wisconsin, Michigan, and Maine, accounts for 0.7% of the total study area. Overall, the means of the USDA ground-based measurements and the TOMS data are 7.8 and 8.6, respectively. The standard deviations of the UV-I values based on these two data sources are 11.42 and 13.75, respectively. As in the spring, it can be concluded from the range of values associated with both datasets that the summertime TOMS data may exhibit more versatile spatial patterns in response to the terrain complexity.

The UV-I spatial distribution maps for fall (see Fig. 10.6) portray very similar patterns across the continental U.S. for both data sources (i.e., USDA and TOMS).

Figure 10.6 Map of the UV-I spatial distributions in fall based on (a) USDA ground-based measurements and (b) TOMS data

In summary, based on the USDA ground-based measurements, the maximum of the UV-I in fall is 6.4. The region spreading from the south of New Mexico, where the UV-I values exceed 6, accounts for 0.8% of the total study area. The maximum of the UV-I in fall is 7.7 based on the TOMS satellite data. The region covering southern Arizona, New Mexico, Texas and Florida, where the UV-I values exceed 6, accounts for 10.1% of the total study area. The minimum of the fall USDA ground-based measurements data is 2.1, and the area where the UV-I values are less than 2.0 accounts for 22.5% of the total study area. On the other hand, the minimum of TOMS satellite data in fall is 2.0, and the area where the UV-I values are less than 2.0 accounts for 18.7% of the total study area, located mostly in the northern part of the continental U.S. Overall, the means of the USDA ground-based measurements and the TOMS data are 3.8 and 4.3, respectively. The standard deviations of the UV-I values, based on these two data sources, are 9.25 and 12.43, respectively. Again, the TOMS data appear to be more sensitive to spatial patterns in response to the terrain complexity.

The two UV-I spatial distribution maps in winter (see Fig. 10.7) are also very similar. In summary, based on the USDA ground-based measurements, the maximum of the UV-I in winter is 4.8. The region located in the south of Texas and Florida, where the UV-I values exceed 4, accounts for 0.6% of the total study area. The maximum of the UV-I in winter is 5.9 with the TOMS satellite data. The region spreading across Arizona, New Mexico, Texas and also southern Florida, where the UV-I values exceed 4, accounts for 2.9% of the total study area. The minimum of USDA ground-based measurements data in winter is 0.9 and the region spreading from Minnesota and Michigan, to New York, where the UV-I values are less than 1.0, accounts for 2.7% of the total study area. On the other hand, the minimum of TOMS satellite data in winter is 0.3 and the region mostly located in the northern part of the continental U.S., where the UV-I values are less than 1.0, accounts for 22.5% of the total study area. Overall, the means of the USDA ground-based measurements and the TOMS data are 2.0 and 2.04, respectively. The standard deviations of the UV-I values based on these two data sources are 6.69 and 10.55, respectively. As with the rest of the year, TOMS data are better able to capture spatial variation in UV-I in response to topography.

Across all seasons, and for both USDA ground-based measurements and TOMS data, the distribution of the UV-I appears to be strongly tied to latitude and topography simultaneously. The higher the latitude, the smaller the UV-I value (Fig. 10.8). The maxima of seasonal and yearly UV-I values are distributed along the latitudes of Arizona, New Mexico, Texas, and southern Florida whereas the minima of the values are distributed across the upper latitudes of the Great Lakes and the Central Plains regions. The UV-I values were also greatly influenced by the topography from east to west. Along the same latitude, the UV-I value in the east is normally smaller due to lower altitudes, while the west is larger due to

(a) US DA ground - based measurements

Figure 10.7 Map of the UV-I spatial distributions in winter based on (a) USDA ground-based measurements and (b) TOMS data

Figure 10.7 Map of the UV-I spatial distributions in winter based on (a) USDA ground-based measurements and (b) TOMS data higher altitudes. Overall, as a result of the combination of the effects of both latitude and altitude, the UV-I distribution pattern shows a characteristic trend of high values in the southwest and low values in the northeast.

On average, the UV-I values based on TOMS data are 1 - 2 units larger than those based on USDA ground-based measurements. The spatial variation of TOMS data is much more evident and is less generalized than the ground-based counterpart. TOMS data can respond to the topography and latitude remarkably and can easily embody the spatial distribution patterns and characteristics of UV-I. Both types of data accurately depict the macroscopic spatial distribution pattern of UV-I in the continental U.S., but TOMS better captures the coverage of spatial patterns. In any circumstance, the spatial comparisons described above are not intended to be indicative of the overall accuracy of either dataset.

{a) USDA ground-based measurements

Figure 10.8 Maps of UV-I spatial distributions based on a multi-year average

Figure 10.8 Maps of UV-I spatial distributions based on a multi-year average